It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that "shafaq" means the redness. Sayyiduna Umar (may Allah be pleased with him), Ali (may Allah be pleased with him) and Abu Hurayrah (may Allah be pleased with him) say that "shafaq" means the redness.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1745